Hello all!I'm not sure should this be in Machine Shop or here. Please move if it's in wrong place.
I've started to design soundproof enclosure for microphone noise measuring purposes based on Neumann's isolationtube like one in the picture below.
I have ocassional access to great mic lab but I'd like to have one myself and due very simple construction of enclosure I decided to build it. I found source for steel tube with outer diameter 150mm and inner dia of 80mm which leaves 35mm of steel in walls -should stop pretty much everything. Top and bottom plates are solid 150mm round bar which is going into lathe for re-shaping. Bottom plate is fixed with reasonable amount of 6mm or 8mm screws, seal between plate and edge of tube. Top plate has quick access through 50mm hole or for bigger microphones quick-lock system made pair of toggle clamps.
Below is simple 3D pictures of parts I've designed this far.
For accurate A-weighted noise measures I also should have ability to use constant SPL source to measure microphones sensitivy, otherwise I can only measure millivolts coming out from circuit. So I thought it might
be possible include constant SPL source inside of tube. I found Tang Band element
small enough it fits in bottom plate and it's able to produce 94dBspl@1kHz with few watts and it dosen't cost fortune, like 30€ for pair.
What happens inside of tube at 1kHz? Is it constant? I just can't figure it out. Something is telling me it might work but on the other hand it's after all solid steeltube full of reflections so there's good change for cancellation points. How much distance from element will effect?
I also thought to use bigger diameter tube - like 200mm - and fill it with pipe of rockwool but what happens when I measure side access mics?
All your thoughts are very much appreciated.